The present disclosure is directed generally to microeletromechanical systems (MEMS) and, more particularly, to MEMS beams, actuators and devices built therefrom.
Various micro-actuation techniques such as electrostatic, thermal, piezoelectric, or magnetic have been demonstrated. Actuators based on electrostatic forces have been commonly used, due to their low power and high frequency operation. Although electrostatic actuators have these advantages, they require high voltages (>40V) that are not compatible with most integrated circuit processes. The maximum forces produced by electrostatic actuators are generally in the range from up to 10 μN, which is lower than the forces produced by other types of microactuators. Large areas are needed for electrostatic actuator designs, which make system on-chip integration less feasible economically. Magnetic actuation uses the force of attraction and repulsion between a magnetic field produced by an electric current and a magnetic material. These types of actuators require extra fabrication steps. Piezoelectric actuators also have similar problems with processing complexity, as they require piezo-electric materials modified by high temperature steps. On the other hand, devices based on electrothermal actuation can provide large forces, large displacements, and low area consumption. They can also operate in an integrated circuit voltage regime (<5V). However, thermal actuators consume more power than electrostatic actuators. Generally thermal actuators are slower than the electrostatic actuators. Usually thermal time constants are longer than the electrical and mechanical time of constants. To alleviate this problem, the thermal mass of the actuators should be designed as small as possible.
Some of the early electrothermal actuator designs are based on the bimorph effect, which relies on the difference of thermal expansion coefficients between two adjacent layers on the device. By heating these layers, a bending moment is created. However such actuators produce deflection in the direction normal to the substrate. One of the microactuator designs by Reithmuller and Benecke with 2.5 μm thick locally deposited Au layer achieved 90 μm displacement by using 200 mW power from 0.05 mm2 area. An electrothermal design by Sun and Carr uses the out of plane actuators to produce in-plane deflections. By using both electrothermal and electrostatic actuation at the same time, this actuator design can produce 30 μm lateral deflection with 40 mW power from 0.03 mm2 area. Because of the fact that processing adjacent bimorph materials is so complicated, the lateral actuation mechanism is very difficult to achieve by using the bimorph approach. Judy et al. developed an actuator which achieved in plane actuation by using serpentine shaped actuators with complicated processing. One recent actuator design by Oz and Fedder uses the CMOS/BICMOS interconnect stacks for laying the bimorph materials to make the processing easier and to also achieve lateral deflections. This actuator demonstrated 3.5 μm deflection by using 18 mW power from 0.04 mm2 area. Lateral “heatuator” microactuators are based on the asymmetrical thermal expansion of a microstructure, which has two different cross sections and is processed in one structural layer. The most recent design by Comtois, Michalicek and Baron can produce 20 μm deflection and 19 μN force with 37 mW power from a small area (0.01 mm2). The 3-db bandwidth for this design is 7 kHz, and maximum frequency for full deflection is 1.57 kHz. For the beam-bent actuators designed by Gianchandani et al., current is passed through the V-shaped beam anchored at two ends to cause a thermal expansion at the center of the actuator. A fabricated single device can produce 5 μm displacement and 8300 μN force with 180 mW from 0.01 mm2 area, and some cascaded ones demonstrated 3 μm deflection and 132 μN force with 40 mW from 0.7 mm2 area. The trade-off between power-area and force can be seen in these actuators. The measured −3 dB bandwidth for both cascaded and single devices is 700 Hz. To increase the output displacements, rotary actuators and inchworm designs are demonstrated by using multiple bent-beam thermal actuators orthogonally. For rotary actuator designs, the displacement is increased from 3 μm to 33 μm, but the power is also increased from 40 mW to 375 mW as multiple actuators are used. Zero-standby power is achieved by the inchworm designs, which means that the power is only needed during the switching time, not for the on or off cases. Sun, Farmer and Carr developed a similar zero-standby operation design by employing a mechanical latch structure. For near zero-power operation, a RF MEMS switch by Robert et al. is designed by a combination of thermal actuation and electrostatic latching. 400 mW of power is consumed for switching operation, but only 10 V is needed for the electrostatic latch mechanism with close to zero continuous power. The switching time for the electrothermal actuation is 200 μs.
There are several examples of applications in MEMS utilizing electrothermal actuators including: RF MEMS tunable capacitors, RF MEMS switches, an optical fiber micro switch, rotary micro-engines, micro-tweezers, and positioners.
Fabrication steps of integrated MEMS, compatible with the present microactuators, may use CMOS post-processing techniques. Structures are made using the CMOS interconnect stack and released with a maskless CMOS micromachining process. The high-aspect-ratio CMOS micromachining technology begins with a conventional foundry CMOS process. After the foundry fabrication, three dry-etch steps, shown in FIG. 1, are used to define and release the structure. FIG. 1(a) shows the cross section of the chip after regular CMOS fabrication. In the first step of post-CMOS processing (FIG. 1(b)), dielectric layers are removed by an anisotropic CHF3/O2 reactive ion etch (RIE) with the top metal layer acting as an etch resistant mask. After the sidewall of the microstructure is precisely defined, silicon trenches around the device are micromachined into the substrate using a deep RIE step (FIG. 1(c)). The final step is an isotropic SF6/O2 RIE used to etch away the bulk silicon and release the structure (FIG. 1(d)). Multi-layer conductors can be built in the composite structure, which enables more flexible designs than homogeneous conducting structures. The undercut of silicon in the release step (FIG. 1(d)) suggests placing the sensing circuits at least 40 μm away from the microstructures. A modified process flow uses photoresist or other material to mask the dielectric layer etch step, instead of using the top metal layer.
In prior work, Lakdawala et al made an infrared sensor from two in-plane bi-material beam elements. A stress gradient is created by heating the beams with infrared radiation and is caused from the change in temperature coefficient of expansion of the materials within the beam. The stress gradient produces a bending moment in each beam and causes the beams to move apart. The air-gap capacitance between the beams changes from the beam motion and is detected with a capacitive sensing circuit.
Over the past decade, MEMS technology has been widely used in applications such as optical communications, wireless systems, automotive sensors, aerospace systems, micro-robotics, chemical sensors, biotechnologies, and micro probes. MEMS applications in the RF and microwave field have seen an incredible growth over the last decade stemming from the superior high frequency performance of RF MEMS switches. During those years, other RF and microwave MEMS devices have been designed such as tunable capacitors, inductors, micro-machined transmission lines, micro-mechanical resonators and filters.
For wireless industries, there is a continuing demand for RF high performance transceivers with lower-power, lower noise and smaller footprint. It is important to use high-quality factor (Q) passive components such as inductors, tunable capacitors and switches in RF front-end circuits for low power and low noise receivers. For oscillators and amplifiers, using a passive component with high-Q results in better phase noise and power consumption. The quality factor of on-chip inductors and MOS varactors is only on the order of low 10s at higher frequencies, therefore off-chip passive components capable of higher Q are widely used for RF front-end circuits. However, using an off-chip device increases the footprint of the receiver. Recent MEMS-based passive components achieved Qs of 30-100 at several gigahertz frequencies and have the potential to be used instead of the low-Q conventional on-chip passives. RF front-ends with these micro-machined passives still have large footprints, because they employ two separate die, one for micro-machined passives and one for electronics. On-chip MEMS passives are of interest, if they can be demonstrated to achieve higher Qs and smaller footprints from the same design.
There is also an increasing demand for multi-band radio architectures, because of the need for integration of different wireless systems with different operation frequencies. Tunable or reconfigurable receiver components are required for these multi-band RF front-ends. Most of the on-chip varactors have low tuning range (<3) and non-linear behavior. Over the past years, MEMS-based tunable capacitors also achieved large tuning ranges (>8) and linear behavior, but previous VCO designs with micromechanical tunable capacitors have not achieved wide tuning for VCO application. On-chip interconnects introduce fixed capacitance to the LC tank of the VCO, which decreases the tuning range
Complete receiver systems on a single chip require voltage-controlled oscillators (VCOs) with gigahertz frequencies, and low phase noise and tunable RF filters with low insertion loss. Tunable capacitors with high Q are desired in VCOs and RF filters for achieving better performance. Micro-mechanical high-Q tunable capacitors have been used for VCO and RF filter applications. Other than the MEMS-based tunable capacitors, several other strategies, which include the implementation of MOS varactors or switched capacitor banks, have been used to achieve wide tuning range. Distortion and linearity are the two main problems associated with these approaches. Compared with solid-state varactors, MEMS tunable capacitors have advantages of lower loss, larger tuning range and more linear tuning characteristics.
In the past few years, many tunable capacitors based on MEMS technology have been designed. These capacitor designs can be classified into two categories according to their tuning mechanism; one category is gap tuning, and the other one is area tuning. MEMS-based RF tunable capacitors can also be classified according to their actuating mechanisms which are; electrostatic, electrothermal, and piezo-electric, discussed above.
Some of the early gap tuning designs have low tuning ranges, because of the parasitic capacitances coming from interconnects. The parallel-plate capacitor designs with electrostatic actuators have a theoretical 50% tuning range limitation, because the electrodes snap after the gap between them becomes ⅔ of the initial gap. The parallel-plate vertical gap device demonstrated by Young and Boser has a tuning range of 16% and quality factor of 60 at 1 GHz. A VCO is implemented at 714 MHz operating frequency with 14 MHz tuning range and a phase noise of −107 dBc/Hz at 100-kHz offset. A modified parallel-plate RF tunable capacitor is designed to increase the tuning range larger than the 50% limit by using three parallel plates. From a 4 pF capacitor design, a tuning range of 87% with 4.4 V controlling voltage and Q of 15.4 at 1 GHz are achieved. A VCO is also demonstrated with 24 MHz tuning at 1.336 GHz operating frequency and phase noise of −98.5 dBc/Hz at 100-kHz offset. A parallel plate design by Zou et al. used a novel electrode design to achieve a tuning range larger than the 50% snap-in limit. For this design, larger gaps are used in the electrostatic actuation mechanism, compared to the gaps between the electrodes of the capacitor. A tuning range of 69% is achieved by using 17 V driving voltages. Designs based on a cantilever beam also achieve tuning ranges larger than 50%. The initial design by Hung and Senturia has a tuning range of 81.8% with 40V controlling voltage. Later designs have Q of 4 at 3 GHz and large tuning range of 354% with 40 V controlling voltage. Parallel-plate capacitor designs using electrothermal and piezo-electric actuation do not have the 50% tuning range limitation. A parallel-plate capacitor by Feng et al. based on thermal actuation has lower driving voltages around 7 V, compared to the capacitor based on electrostatic actuation. It has also large tuning ranges of 270% and high-Q factor of 300 at 10 GHz. Yao et al. developed a capacitor based on piezo-electric actuation that has a Q factor of 210 at 1 GHz with a 6 V controlling voltage.
The area tuning RF MEMS capacitors are demonstrated to solve the snap-in tuning range limitation, when electrostatic actuation is used. Early devices achieved a tuning range of 300% with 5 V controlling voltage. Interdigitated finger structures with 30 μm thickness are used for capacitor electrodes and the electrostatic actuation mechanism. Recent designs use thicker and longer finger blocks to increase the tuning range and the quality factor. Tuning ratio of 8.4:1 with 8 V controlling voltage and Q factor of 35 at 2 GHz is demonstrated by using 40 μm thick finger electrodes. Having a 12 pF nominal capacitance value and Q-factor of 200 at 400 MHz enables UHF filter applications for these capacitor designs. A UHF filter with tuning range of 225-400 MHz is developed with an insertion loss of 6.2 dB and Q factor of 100 for the operating frequency ranges. The most recent capacitor design by Rockwell Science Center achieves a linear tuning characteristic by forming a completely electrically isolated capacitor. Two sets of electrostatic comb drive actuators in opposing directions are used to achieve a linear tuning characteristic. Another area tuning design uses the idea that the dielectric between the electrodes is moved laterally to achieve high-Q factors. Using a dielectric that has large dielectric constant enables high-Q factors, because the same device would have bigger capacitance with the same resistance losses. Q factors of 291 at 1 GHz and a tuning range of 7% with 10 V controlling voltage is demonstrated. The biggest issue of these MEMS devices is the use of separately fabricated CMOS/BICMOS electronics. For VCO and RF filter designs, on-chip and off-chip interconnects between separate dies introduce large fixed capacitance to the LC tank, which decreases the tuning range.